1. Field of the Invention
The present invention relates generally to optical amplifiers used in fiber-optics for telecommunications, cable television and other fiber-optics applications, and more particularly to an optical fiber amplifier and method for producing an amplified broadband output from an optical signal with dispersion compensation.
2. Description of the Related Art
Because of the increase in data intensive applications, the demand for bandwidth in communications has been growing tremendously. In response, the installed capacity of telecommunication systems has been increasing by an order of magnitude every three to four years since the mid 1970s. Much of this capacity increase has been supplied by optical fibers that provide a four-order-of-magnitude bandwidth enhancement over twisted-pair copper wires.
To exploit the bandwidth of optical fibers, two key technologies have been developed and used in the telecommunication industry: optical amplifiers and wavelength-division multiplexing (WDM). Optical amplifiers boost the signal strength and compensate for inherent fiber loss and other splitting and insertion losses. WDM enables different wavelengths of light to carry different signals parallel over the same optical fiber. Although WDM is critical in that it allows utilization of a major fraction of the fiber bandwidth, it would not be cost-effective without optical amplifiers. In particular, a broadband optical amplifier that permits simultaneous amplification of many WDM channels is a key enabler for utilizing the full fiber bandwidth.
Silica-based optical fiber has its lowest loss window around 1550 nm with approximately 25 THz of bandwidth between 1430 and 1620 nm. For example, FIG. 1 illustrates the loss profile of a 50 km optical fiber. In this wavelength region, erbium-doped fiber amplifiers (EDFAs) are widely used. However, as indicated in FIG. 2, the absorption band of a EDFA nearly overlaps its the emission band. For wavelengths shorter than about 1525 nm, erbium-atoms in typical glasses will absorb more than amplify. To broaden the gain spectra of EDFAs, various dopings have been added. For example, as shown in FIG. 3a, codoping of the silica core with aluminum or phosphorus broadens the emission spectrum considerably. Nevertheless, as depicted in FIG. 3b, the absorption peak for the various glasses is still around 1530 nm.
Hence, broadening the bandwidth of EDFAs to accommodate a larger number of WDM channels has become a subject of intense research. As an example of the state-of-the-art, a two-band architecture for an ultra-wideband EDFA with a record optical bandwidth of 80 nm has been demonstrated. To obtain a low noise figure and high output power, the two bands share a common first gain section and have distinct second gain sections. The 80 nm bandwidth comes from one amplifier (so-called conventional band or C-band) from 1525.6 to 1562.5 nm and another amplifier (so-called long band or L-band) from 1569.4 to 1612.8 nm. As other examples, a 54 nm gain bandwidth achieved with two EDFAs in a parallel configuration, i.e., one optimized for 1530-1560 nm and the other optimized for 1576-1600 nm, and a 52 nm EDFA that used two-stage EDFAs with an intermediate equalizer have been demonstrated.
These recent developments illustrate several points in the search for broader bandwidth amplifiers for the low-loss window in optical fibers. First, bandwidth in excess of 40-50 nm require the use of parallel combination of amplifiers even with EDFAs. Second, the 80 nm bandwidth may be very close to the theoretical maximum. The short wavelength side at about 1525 nm is limited by the inherent absorption in erbium, and long wavelength side is limited by bend-induced losses in standard fibers at above 1620 nm. Therefore, even with these recent advances, half of the bandwidth of the low-loss window, i.e., 1430-1530 nm, remains without an optical amplifier.
There is a need for nonlinear polarization amplifiers that provide a low noise figure amplification for operation near the zero dispersion wavelength of fibers. There is a further need for a broadband fiber transmission system that includes nonlinear polarization amplifiers which provide low noise amplification near the zero dispersion wavelength of fibers.
Accordingly, an object of the present invention is to provide a broadband nonlinear polarization amplifier.
Another object of the present invention is to provide a broadband nonlinear polarization amplifier with a distributed gain medium.
A further object of the present invention is to provide a broadband nonlinear polarization amplifier that has a distributed gain medium with a magnitude of dispersion that is less than 50 ps/nm-km.
Yet another object of the present invention is to provide a broadband nonlinear polarization amplifier with a transmission line that includes a Raman amplifier, and at least a portion of the transmission line with a magnitude of dispersion less than 50 ps/nm-km.
Another object of the present invention is to provide an amplifier module that includes a dispersion compensating fiber with a negative sign of dispersion and an absolute magnitude of dispersion of at least 50 ps/nm-km.
A further object of the present invention is to provide an amplifier module that has a transmission fiber and a dispersion compensating fiber, where a difference between the relative dispersion slopes of the transmission fiber and the dispersion compensating fiber is no greater than 0.0032/nm over at least a portion of a signal wavelength range.
Yet another object of the present invention is to provide an amplifier module that includes a dispersion compensating fiber a pump source that produces a depolarized pump beam.
Still a further object of the present invention is to provide an optical fiber communication system that includes a dispersion compensating fiber with at least a portion having a negative sign of dispersion and an absolute magnitude of dispersion of at least 50 ps/nm-km.
These and other objects of the present invention are achieved in a broadband nonlinear polarization amplifier with an input port for inputting an optical signal having a wavelength xcex. A distributed gain medium receives and amplifiers the optical signal through nonlinear polarization. The distributed gain medium has zero-dispersion at wavelength xcex0. A magnitude of dispersion at xcex is less than 50 ps/nm-km. One or more semiconductor lasers are operated at wavelengths xcexp for generating a pump light to pump the distributed gain medium. An output port outputs the amplified optical signal.
In another embodiment of the present invention, a broadband fiber transmission system includes a transmission line having at least one zero dispersion wavelength xcexo and transmitting an optical signal of xcex. The transmission line includes a Raman amplifier that amplifies the optical signal through Raman gain. At least a portion of the transmission line has a magnitude of dispersion at xcex less than 50 ps/nm-km. One or more semiconductor lasers are operated at wavelengths xcexp and can generate a pump light to pump the Raman amplifier. xcex is close to xcex0 and xcex0 is less than 1540 nm or greater than 1560 nm.
In another embodiment of the present invention, a broadband fiber transmission system includes a transmission line having at least one zero dispersion wavelength xcexo, and transmitting an optical signal of xcex. The transmission line includes a Raman amplifier and a discrete optical amplifier that amplify the optical signal. At least a portion of transmission line has a magnitude of dispersion at xcex less than 50 ps/nm-km. One or more semiconductor lasers are operated at wavelengths xcexp and can generate a pump light to pump the amplifiers. xcex is close to xcex0 and xcex0 is less than 1540 nm or greater than 1560 nm
In another embodiment of the present invention, an amplifier module includes a transmission fiber configured to transmit a signal. A dispersion compensating fiber has at least a portion with a negative sign of dispersion and an absolute magnitude of dispersion of at least 50 ps/nm-km. A first intermediate fiber couples the dispersion compensating fiber with the transmission fiber. The first intermediate fiber has a mode field diameter that is less than a mode field diameter of the transmission fiber and greater than a mode field diameter of the dispersion compensating fiber. At least a first pump source is coupled to the transmission fiber. The first pump source produces a first pump beam that creates Raman gain in the dispersion compensating fiber.
In another embodiment of the present invention, an amplifier module includes a transmission fiber has a relative dispersion slope and is configured to transmit a signal. A dispersion compensating fiber has a relative dispersion slope and is coupled to the transmission fiber. A difference between the relative dispersion slopes of the transmission fiber and the dispersion compensating fiber is no greater than 0.0032/nm over at least a portion of a signal wavelength range. At least a first pump source is coupled to the transmission fiber and produces a first pump beam that creates Raman gain in the dispersion compensating fiber.
In another embodiment of the present invention, an amplifier module includes a transmission fiber configured to transmit a signal. A dispersion compensating fiber is coupled to the transmission fiber. At least a first pump source is coupled to the transmission fiber. The first pump source produces a depolarized first pump beam that creates Raman gain in the dispersion compensating fiber.
In another embodiment of the present invention, an optical fiber communication system includes a transmitter, a receiver and a transmission fiber coupled to the transmitter and the receiver. The transmission fiber exhibits chromatic dispersion at a system wavelength. A dispersion compensating fiber is also included. At least a portion of the dispersion compensating fiber has a negative sign of dispersion and an absolute magnitude of dispersion of at least 50 ps/nm-km. A first intermediate fiber couples the dispersion compensating fiber with the transmission fiber. The first intermediate fiber has a mode field diameter that is less than a mode field diameter of the transmission fiber and greater than a mode field diameter of the dispersion compensating fiber. At least a first pump source is coupled to the transmission fiber and produces a first pump beam that creates Raman gain in the dispersion compensating fiber.